An apparatus is known (DE-OS 33 06 720, U.S. Pat. No. 4,541,173) in which an X-ray film cassette is positioned between two parallelly guided bars pulled together by a tension spring and is transported by means of motor-driven rollers on a receiving plate towards an end abutment in a cassette unloading and reloading station. The bars have vertically shiftable claws at their free ends located in the area of the end abutment, said claws serving to open the locking means of the upper cassette portion, when the receiving plate and thus the X-ray film cassette are moved to an inclined position, as well as to hold down the lower cassette portion on said receiving plate.
An infrared-light sensor with an associated light-receiving element is arranged above the front area of the X-ray film cassette when said cassette is in its functional position, the infrared light rays of that sensor sensing a point on the inner surface of the lower cassette portion when the upper cassette portion is pivoted to its open position in order to ascertain whether a film sheet is present or not. If there is no film sheet present, the X-ray film cassette is fed out of the apparatus in its open condition.
This known apparatus is disadvantageous in that when the upper cassette portion is pivoted to its open position, the film sheet adheres to the inner surface of the upper cassette portion owing to static charges of the film sheet in surface-to-surface contact with the inner surface of the plastic lining of the upper cassette portion. The film sheet is only released after a period of time and then drops back into the lower cassette portion. If during the adherence of the film sheet to the upper cassette portion in its open position, the lower cassette portion is sensed by infrared light, no film sheet is detected in the lower cassette portion so that the X-ray film cassette in its open position along with the film sheet adhering to the upper cassette portion may be erroneously fed out. If several sensing operations were performed at intervals, with the last one ascertaining that a film sheet was present, having been released from the upper cassette portion in the meantime and dropped back into the lower cassette portion, this operation would take much too long.
If a film sheet of small size is erroneously loaded in an X-ray film cassette of large size the film sheet will be detected even if only one single point in the lower cassette portion is sensed. However the detection does not reveal whether it is of a size suitable for the X-ray film cassette.
Finally, the infrared-light sensor is arranged at a great distance above the X-ray film cassette. This remote arrangement of the infrared-light sensor is necessary because the sensor would otherwise be contacted, and damaged by the upper cassette portion when said portion is pivoted to its open position.